drm/via: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/c21a588bf9e222826f6b138db91de26a2b21df33.1605896060.git.gustavoars@kernel.org

drivers/gpu/drm/via/via_irq.c

@@ -364,6 +364,7 @@ int via_wait_irq(struct drm_device *dev, void *data, struct drm_file *file_priv)
 		irqwait->request.sequence +=
 			atomic_read(&cur_irq->irq_received);
 		irqwait->request.type &= ~_DRM_VBLANK_RELATIVE;
+		break;
 	case VIA_IRQ_ABSOLUTE:
 		break;
 	default: